The present invention relates generally to power conditioning and, more particularly, to a system and method of controlling a motor drive.
Motor drives are frequently used in industrial applications to condition power and otherwise control electric driven motors such as those found with pumps, fans, compressors, cranes, paper mills, steel mills, rolling mills, elevators, machine tools, and the like. Motor drives typically provide a volts-per-hertz control and have greatly improved the efficiency and productivity of electric driven motors and applications. Increasingly, motor drives are taking the form of adjustable-speed drives (ASD) that are adept at providing variable speed and/or variable torque control to an electric driven motor. Heretofore, motor drives have been used solely to control or otherwise condition power to a motor load.
Power to non-motor loads is usually controlled through a power conditioner that is specifically designed to handle the transient load conditions that can be encountered. However, current power conditioners often trip, or cause the load to trip, when strict voltage and current constraints are not met. That is, for non-motor loads, a transformer is often connected between the power conditioner and the load and is used to account for any drops in voltage that occur during conditioning by the power conditioner. At start-up of the power conditioner, it is not uncommon for the transformer to become saturated and, thus, fail as a result of SCR failure or control problems.
That is, it is not uncommon for a DC bias to develop in the transformer. This occurs when one side of the sine wave input to the transformer becomes larger than other. For example, given that one SCR is firing at 60% and the other SCR is firing at only 40%, a 20% bias forms on the first cycle. Since the bias is additive, a 40% bias forms on the second cycle, 60% of the third, 80% of the fourth, and 100% on the 5th. As a result, the transformer becomes saturated; although, symptoms of saturation may begin to surface at 70% saturation.
This saturation can cause damage to the transformer. Specifically, the polarity of the primary and secondary windings becomes the same, i.e. the transformer core becomes biased to one side only and the windings take the other side. With the same polarity, the primary and secondary windings try to force themselves away from one another. If this condition is maintained, the transformer will fail.
Transformer saturation also yields extremely high primary currents coupled with a decrease in secondary voltage. Under normal conditions, the dominant load on the primary line is the secondary winding of the transformer. Once the transformer is saturated, the transformer core also begins to act as a load. As the core loads the primary winding, the secondary winding becomes less of a load and, as a result, a drop in secondary voltage can be observed.
Also, given that saturation increases exponentially, at total saturation, the transformer core is unable to present any more of a load to the line. As such, little, if any, secondary current is present. In this regard, the primary winding becomes a short circuit which will either trip a circuit breaker or blow a fuse in the system. In any event, undesirable system failure occurs.
Saturation of the transformer can be particularly problematic during start-up of the power conditioner. Power conditioners typically include a series of switches, such as IGBTs, that are switched at high frequencies, e.g., 10 kHz, to provide a desired output voltage that is seen by the load. Further, power conditioners operate according to a volts-per-hertz (V/Hz) profile such that voltage changes can be exacted by varying the duty cycle and fundamental frequency of the IGBTs or other switch circuit. During start-up, the controller will control the IGBT switching duty cycles such that the power conditioner quickly ramps up to steady-state levels. More specifically, when the power conditioner is turned ON, it is generally desirable to control the power conditioner to provide a steady-state output (voltage and frequency of operation) as quickly as possible. As such, the V/Hz curve along which the power conditioner operates is adhered to during steady-state operation, but ignored during start-up. As a result, the power conditioner may provide an output to the transformer that causes a DC bias to develop in the transformer. As described above, this DC bias can cause transformer saturation and, ultimately, component or system failure. That is, flux may develop in the transformer core that causes transformer saturation. It is generally recognized that the flux in a transformer can be defined by the following expression:
            B      max        =                  E        RMS                    4.44        ·        S        ·        N        ·        f              ,(Eqn. 1), where:    Bmax Core induction (T)    ERMS RMS voltage (V)    S Core cross section (m2)    N Number of Turns    f Frequency (Hz).As “S” and “N” are fixed for a given transformer, only two variable “ERMS” and “f vary and must be controlled. Thus, if voltage increases at a much faster rate than frequency then saturation can occur. In other words, voltage and frequency must be matched to maintain a desired flux level.
It would therefore be desirable to have a cost-effective system and method of controlling a power conditioner connected a non-motor load to prevent transformer saturation during power conditioner start-up.